Conventionally, a power unit that is capable of compensating for a voltage drop at a load due to wiring without using two remote sensing lines for low cost has been available, for example, as disclosed by Patent Reference 1.
Further, in order to perform phase compensation for a constant-voltage circuit, conventionally, a capacitor is often provided at the output terminal of the constant-voltage circuit in parallel to the load as shown in FIG. 3. An internal impedance of ESR and a capacitance of a capacitor C101 provide the phase compensation, and improve the frequency characteristics of the constant-voltage circuit by moving a pole and generating a zero point in the frequency characteristics. Since the advantage of this method is that the constant-voltage circuit does not have to provide a terminal for phase compensation, the number of terminals of a power supply IC can be small. For such phase compensation method, a tantalum capacitor having a great ESR is normally used.
As shown in FIG. 4, the typical ESR of a tantalum capacitor having a capacitance of 2.2 μF ranges from 1 Ω to 10 Ω, which ESR provides the zero point at a desirable region in the frequency characteristics of the constant-voltage circuit for phase compensation, and accordingly, satisfactory phase compensation is available. Nevertheless, recently and continuing, ceramic capacitors that are smaller and lighter-weight than tantalum capacitors, having a large capacitance, are available with a stable supply at low cost. Accordingly, requirements for using the ceramic capacitor as the capacitor for the phase compensation are increasing.
Here, the ESR of the ceramic capacitor is small, ranging from 10 mΩ to 30 mΩ, which is 100 to 1000 times smaller than the tantalum capacitor as shown in FIG. 5. Accordingly, when the ceramic capacitor is used for phase compensation, the frequency at which a zero point is obtained moves to a very high frequency, and suitable phase compensation cannot be obtained.
In order to lower the frequency at which the zero point is obtained, a solution may be to insert a resistor in series with the ceramic capacitor, the resister being provided outside of a power supply IC (constant voltage IC). However, it is disadvantageous for space and cost reasons. Accordingly, it is preferred that the resistor be provided inside the power supply IC.
FIG. 6 and FIG. 7 show examples of a circuit where a resistor is provided in the power supply IC.
The example shown in FIG. 6 includes a terminal PinVout2, which is an IC package terminal, for connecting a ceramic capacitor, a fixed resistor R103 having a resistance value of about 100 mΩ for phase compensation provided between a pad ICP2 of the IC chip and the terminal PinVout2, and an output terminal PinVout1 for outputting a voltage. In a case like this, since the output current io does not flow through the fixed resistor R103, the output voltage is stably available.
In the case of the example shown in FIG. 7, the resistance value of the fixed resistor R103 for phase compensation ranges from 100 mΩ to 10 Ω, the resistor R103 being provided between a pad ICP of the IC chip and the output terminal PinVout of the IC.
While in the case of the example shown in FIG. 7, the number of IC terminals is smaller than the example of FIG. 6, the output current io flows through the fixed resistor R103. When the output current io becomes great, a voltage drop Vdrop (=io×resistance of R103) across the fixed resistor R103 cannot be neglected. In order to compensate for the voltage drop Vdrop, a resistor R104 having a fixed resistance value is inserted between a reference voltage source Vref and the grounding voltage, a load is connected between the output terminal PinVout and the resistor R104, and the same output current io flows through the fixed resistor R104 and the load.
According to this arrangement, if the output current io increases, a voltage drop across the fixed resistor R104 increases, and a voltage of the non-inverted input terminal of an error amplifying circuit AMP into which the reference voltage Vref is input rises. For this reason, an internal output voltage Vo of the constant-voltage circuit is raised, and the voltage drop Vdrop due to the fixed resistor R103 is compensated for. In order to completely remove the influence of the fixed resistor R103, relations between resistors R101 and R102 for output voltage detection, and the fixed resistors R103 and R104 are set as(resistance of R101)/(resistance of R102)=(resistance of R103)/(resistance of R104).
However, if (resistance of R101)/(resistance of R102)<(resistance of R103)/(resistance of R104), positive feedback starts occurring, and the output voltage rises. Accordingly, the relations are usually made into(resistance of R101)/(resistance of R102)>=(resistance of R103)/(resistance of R104).
[Patent Reference 1]. JPA 10-257764